There are two basic types of bipolar transistors: PNP and NPN. One bipolar transistor consists of two back-to-back PN junctions. In such a three-layered semiconductor, the intermediate layer is referred to as a base area (b), and the left and right layers are referred to as an emitter area (e) and a collector area (c) respectively. An emitter junction is formed between the emitter area and the base area, and a collector junction is formed between the collector area and the base area.
Structures of the bipolar transistor and methods for manufacturing the bipolar transistor have been studied for a long time, and reference can be made to the disclosure of Chinese Patent Application No. 91104429.9 for common structures of the bipolar transistor and methods for manufacturing the bipolar transistor.
Also, there is disclosed in the prior art the structure of a vertical NPN-type bipolar transistor parasitic in the structure of conventional nMOS transistor as illustrated in FIG. 1. The NPN-type bipolar transistor includes a p-type semiconductor substrate 100, a deep n-type doped well 101 (DNW) arranged in the semiconductor substrate 100, a p-type doped well 102 (PW) arranged in the semiconductor substrate 100 and surrounded by the deep n-type doped well 101, and an n+ doped area in the semiconductor substrate 100 to form the source and the drain of the nMOS transistor. The deep n-type doped well 101, the p-type doped well 102 and the n+ doped area form an NPN bipolar transistor. Of course, the conventional nMOS transistor further includes a gate dielectric layer 103 and a polysilicon gate 104.
Similarly, a vertical parasitic PNP-type bipolar transistor can be formed in the structure of a pMOS transistor.
In the above NPN-type bipolar transistor, the deep n-type doped well 101 serves as a collector electrode, the p-type doped well 102 serves as the base, and the n+ doped area serves as the emitter electrode. Electrodes are formed respectively and connect to the emitter electrode, the base, and the collector via contacts and then a current source is applied to the base in order to operate the bipolar transistor for turn-on or off.
Moreover, a precise and stable voltage reference circuit is required in various integrated circuits, e.g. a Digital to Analogy Converter (DAC), an Analogy to Digital Converter (ADC), a linear manostat, a switch manostat, etc. The reference voltage can influence directly the performance of integrated circuits and thus requires good stability and precision. The band-gap reference circuit is one of the widely used circuits with low temperature coefficient.
The characteristic voltage inherent to the silicon material itself (i.e. the silicon band-gap) is widely used in a reference voltage circuits for high precision. However, the silicon material has a certain temperature coefficient, thus it is common to select another device parameter with a temperature coefficient of opposite polarity and approximately same magnitude (e.g., the ΔVBE circuit), so that by adding the 2 parameters with opposite temperature coefficients together the overall net temperature coefficient can be nearly zero.
The band-gap reference circuit is an elementary circuit offering a stable reference voltage (˜1.2V). Its stability (with respect to temperature) is related to an output voltage which is a combination of VBE (a base-emitter bias voltage) and ΔVBE (the difference between two base-emitter bias voltages) with their temperature coefficients cancelling each other.